Enhanced Cr(VI) removal of zero-valent iron with high proton conductive FeC2O4•2H2O shell

In this study, we coated a high proton conducive shell on the zero-valent iron (ZVI) surface by mechanically ball-milling ZVI with oxalic acid dihydrate (OX-ZVI), and demonstrated that the generated FeC2O4 center dot 2H(2)O shell dramatically improved the Cr(VI) removal rate of ZVI by about 15-80 times. Owing to a higher proton conductivity of FeC2O4 center dot 2H(2)O shell than that of Fe2O3 shell, proton could easily transfer through FeC2O4 center dot 2H(2)O shell into iron core and be reduced to center dot H, accompanying with fast surface-bound Fe2+ generation, resulting in high efficiency of Cr(VI) removal in a wide pH range. Meanwhile, the removed Cr(VI) was deposited on OX-ZVI surface in the formation of FexCr1-x(OH)(3) composites, accompanied by the appearance of typical hollow structure derived from iron core dissolution. This study clarifies the significance of proton transfer on the reactivity of zero-valent iron, and also provides a new strategy to prepare highly active zero-valent iron for Cr(VI) removal.